There is known a technique called lifetime control which is used in order to shorten reverse recovery time of a diode. This technique uses a technology that irradiates charged particles to a wafer, in which a P-layer and a N-layer are stacked, to intentionally generate crystal defects in the N-layer. Carriers are trapped by the crystal defects, and their lifetime thereby terminates. The charged particles are ions or electrons. As a kind of ions, helium ions or protons are typically selected.
Crystal defects will be outlined. Charged particles irradiated to a wafer eject silicon atoms in a crystal structure, Then, vacancies which lack silicon atoms are formed in the crystal structure. These vacancies are called point defects. The point defects are unstable on their own, and are stabilized by binding to other factors. One aspect of stable defects is called multiple-vacancy defects in which multiple point defects bind to one another. Another aspect of stable defects is called complex defects in which the point defects bind to impurities included in a wafer. That is, complex defects are generated by which the point defects bind to the impurities. Here, “impurities” do not mean substances which determine the conduction type of a semiconductor, but means substances which do not contribute to a conduction type. In this specification, the impurities, which do not contribute to a conduction type but contribute to the stabilization of point defects, are referred to as defect stabilizing impurities, for convenience, in order to distinguish them from impurities that determine the conduction type. Boron and phosphorus are typical examples of the impurities that determine the conduction type, and oxygen and carbon are typical examples of the defect stabilizing impurities.
On the other hand, it is known that lifetime is related to the depth of a defect position in a wafer direction (see JP 2007-251003 A and JP 2009-239269 A). Defect positions in a depth direction of a wafer can be adjusted by energy imparted to charged particles that are irradiated.